Photovoltaic systems use solar cells to convert light into electricity. A typical photovoltaic system includes a plurality of components, including photovoltaic cells, mechanical and electrical connections, mountings, and controllers for regulating and/or modifying the electrical current produced by the photovoltaic system.
The following terms are used herein to describe various components and/or operational aspects of photovoltaic systems:
PV photovoltaic
DC direct current
AC alternate current
ISC short circuit current
VOC open circuit voltage
Pmax maximum output power of the solar array
Vmax output voltage of the solar array at maximum output power
Imax output current of the solar array at maximum output power
FIG. 1 is a block diagram of a conventional photovoltaic power system 100. The PV power system 100 includes a solar cell array 101 that comprises a plurality of solar cells (also referred to as photovoltaic cells) that convert light into DC voltage. The solar cells are solid state devices that convert the energy of sunlight directly into electricity by the photovoltaic effect. The solar cell array 101 is coupled to a DC switch 102. The DC switch 102 can be closed to connect the solar cell array 101 to DC capacitor bank 104, or can be opened to disconnect the solar cell array 101 from the DC capacitor bank 104. When the DC switch 102 is closed and the solar cell array 101 is generating power, the solar cell array 101 can provide power to charge the DC capacitor bank 104.
The DC capacitor bank 104 is connected to inverter 105. The inverter 105 converts the DC voltage output from the capacitor bank 104 into 3-phase (or in some cases 2-phase) pulsed AC voltage. The filter 106 converts the pulsed AC voltage output by the inverter 105 into a sinusoidal AC voltage. The sinusoidal AC voltage can then be output to a mains power grid 109. If an AC mains switch 107 is closed, the sinusoidal AC voltage output by the filter 106 is received by the power transformer 108. The power transformer 108 adapts the voltage output by the PV system 100 to the grid voltage. This configuration allows the PV system 100 to output electricity onto the mains grid 109. The voltage output by the photovoltaic system 100 has to be higher than the grid voltage. Inverter 105 may have a mandatory ramp up period where the DC voltage provided by the solar cell array 101 must be gradually ramped up from an initial startup voltage to an operating voltage.
FIG. 2 is a graph illustrating the characteristics of a photovoltaic cell. The graph is a current-voltage (I-V curve) for a typical PV cell. VOC represents the output voltage of the solar cell or array of solar cells where no load is connected to the PV cell or array of cells. In the example illustrated in FIG. 1, when the DC switch 102 is open, VOC represents the output voltage of the solar cell array 101, because the solar cell array 101 is disconnected from the load (the grid 109).
The value ISC represents the current produced by the PV cell or array of cells in the event that there is a short circuit. As can be seen from the graph in FIG. 2, there is a maximum voltage Vmax where the PV cell or array of cells produces maximum power Pmax. In a typical photovoltaic system, such as PV power system 100, the inverter 105 includes a DC voltage controller (not shown) that controls the DC voltage (VDC) provided by the PV cell or array of cells to operates the PV cell or array of cells at the maximum power point. The grid voltage and the Vmax of the PV cell or array of cells typically do not change very quickly, so the DC-voltage controller typically does not have to dynamically respond to rapid changes in these voltages.
The following is a typical process for powering up of a photovoltaic system, such a PV system 100. If the VDC of the solar array is higher than a predetermined threshold voltage (at least higher than the peak transformer output AC-voltage), the PV system controller closes the DC switch 102 between the DC capacitor bank 104 and the solar cell array 101. Once the DC switch 102 has been closed, the solar array 101 is operating as a current source and begins to charge the DC capacitor bank 104 according to the specific photovoltaic cell characteristics of the solar array and the DC voltage level being generated. Once the DC capacitor bank 104 has been charged to VOC, the inverter 105 closes the AC mains switch 107. The peak of the transformer output AC voltage is lower than VOC. At this point, no current is flowing between the solar array and the grid, even though all the switches are closed. Next, the inverter 105 starts to generate AC voltage and the inverter 105 synchronizes its AC output voltage to grid voltage and grid frequency. AC output current of the inverter 105 during this phase of operation is approximately zero. The photovoltaic system is not yet generating power, and the DC voltage is VOC. Once the inverter 105 has begun generating AC voltage and has synchronized its output voltage with the grid voltage, the DC voltage controller of the photovoltaic system begins operation and reduces the DC voltage from VOC to Vmax. Change of the DC voltage is generally a very slow procedure. FIG. 2 illustrates the difference between VOC and Vmax. When the DC voltage is at VOC, the system is not generating power, and when the DC voltage is a Vmax, the system is generating maximum power.
The standard power up process described above has several disadvantages. For example, VDC is one of the key design parameters for photovoltaic systems, especially with respect to the components of the inverter 105 and the DC capacitor bank 104. The current trend for photovoltaic systems is that the VDC has been increased to higher values (e.g., in the range of 1000V). A lower VDC would be preferred from a design standpoint, because the inverter would not have to handle such high voltages. In order to operate at higher voltages, efficiency is compromised. The maximum DC voltage during operation of the inverter is Vmax, and the voltage level between Vmax and VOC is used only during the power up procedure.
One technique that can be used to lower the DC voltage provided by the solar cell array 101 is to introduce a preload into the PV system to lower the voltage provided to the inverter 105 during the ramp up period for the inverter 105. FIG. 3 illustrates a PV system 300 that includes a power up preload 303 (also referred to herein as a power up load). The PV system 300 includes a solar cell array 301, a DC switch 302, a DC capacitor bank 304, an inverter 305, a filter 306, an AC mains switch 307, and a power transformer 308. The power transformer 308 adapts the voltage output by the PV system 300 to the grid voltage. This configuration allows the PV system 300 to output electricity onto the mains grid 309.
The power up sequence for the PV system 300 is slightly different than that of the PV system 100, because PV system 300 includes the preload 303. In PV system 300, the PV system controller closes the DC switch 302 which connects the solar cell array 301 to the power up load 303. The power up load 303 is disposed between the solar cell array 301 and the DC capacitor bank 304. The controller of the PV-inverter 305 then closes the AC mains switch 307 until the DC capacitor bank 304 is charged to Vpower up. The inverter 305 can then start generating AC voltage and the filter 306 begins synchronizing the AC output voltage and to the grid voltage and grid frequency. The power up load 303 is then disabled by the controller of the PV inverter 305, and the DC voltage controller of the PV system 300 being operating to reduce the DC voltage from Vpower up to Vmax in order to generate maximum power for the PV system.
FIG. 4 illustrates the distinction between operating at Vpower up where the power generated by the system equals Ppower up and operating at Vmax where the power generated by the system equals Pmax. FIG. 4 is a graph illustrating the characteristics of a photovoltaic cell that includes the power up operating point. The graph is a current-voltage (I-V curve) for a typical PV cell. Vpower up represents the power up voltage and Ipower up represent the power up current. As can be seen in FIG. 4, the power up voltage falls between the Vmax and the VOC for the PV cell or array of cells.
In conventional PV systems, such as those illustrated in FIGS. 1 and 3, the inverter may be “overdesigned” to allow the inverter to operate at higher voltages such as the VOC of the solar cell array, but this approach this sacrifices efficiency of the inverter. Furthermore, adding a preload to the system as suggested in the alternative implementation illustrated in FIG. 3 can decrease the voltage levels at which the inverter can operate, but this approach adds expense and complexity to the PV system.